Apparatus for sensing the thickness of a moving sheet of film

ABSTRACT

A rotatable capacitance sensor preferably constructed of a generally cylindrical body mounted on bearings to a carriage assembly. The body has a number of parallel, spaced apart conductive plates having edges extending through the exterior body surface, and alternate ones of the plates are connected via a bearing to a conductor in one part of the carriage assembly, and the remaining ones of the plates are connected via a bearing to a second conductor in another part of the carriage assembly.

This is a division of U.S. application Ser. No. 07/764,774 filed Sep.24, 1991.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for measuring thethickness of moving films; more particularly, the invention relates to arotatable sensor which makes rotatable contact with a moving sheet ofthin material. The most notable application of the invention is inconnection with measuring thickness of thin plastic film in sheet form,of the type typically manufactured in blown film machines.

Most prior art thickness gauges used in connection with blown filmmachines utilize contact devices; i.e., a sensor is brought into contactwith a moving sheet of plastic film and film thickness measurements aremade as the film passes the sensor head. Some of these devices aretransversely movable across or around a blown film bubble so as totraverse the entire circumferential surface area of the film bubble. Inother cases, the sensor head is mounted in a fixed position and thebubble itself rotates transversely at a slow turning rate at the sametime as the film moves longitudinally past the head.

Some prior art film thickness measuring devices do not actually contactthe film, but these devices generally suffer from the disadvantage ofpoor accuracy, and such devices are generally unable to make accuratemeasurements of thickness, particularly of very thin films.

Examples of prior art film thickness measuring devices are found inprior patents. U.S. Pat. No. 3,764,899, issued Oct. 9, 1973, disclosesan electrode assembly which is mounted in a fixed position, and a thinplastic film strip is movably passed across a sensing head to obtainmeasurements of film thickness in conjunction with an electronic circuitfor processing the capacitance signals produced thereby. U.S. Pat. No.4,947,131, issued Aug. 7, 1990, discloses a capacitance bar sensordesigned to be placed in contacting relationship with a moving filmsurface, to provide extremely accurate film thickness measurementsindependent of temperature variations. U.S. Pat. No. 3,300,716, issuedJan. 4, 1967, discloses a capacitance sensing apparatus for measuringtransversely across a sheet of moving film, particularly disclosingelectronic circuits for enhancing the signal processing. Variouscircuits for processing signals from a capacitance sensor of the typedisclosed herein are well known from the foregoing and other prior artpatents, and such circuits are not specifically dealt with in thepresent disclosure.

One of the problems with prior art contact sensors is that, because suchsensors inherently must remain in contact with a moving film, they tendto mark or scratch the film surface. Such sensors may be designed tominimize this problem, but in particular film applications such ashigh-quality optical films, even fine hairline scratches on the film maybe cause for concern. Because of the relative differences in velocitybetween the sensor and the film, the problem of film scratching isalways present with sensors of this type.

SUMMARY OF THE INVENTION

The present invention relates to a rotatable sensor for capacitivelymeasuring the thickness of a moving plastic film, wherein the axis ofrotation of the sensor assembly is transverse to the direction ofmovement of the film. A sensor head is placed in contacting relation tothe film and the head is rotatable by virtue of this contactingrelationship, to produce a rate of travel of the head which issynchronized to the rate of travel of the film sheet. The capacitivesensing element of the sensor head is made from a plurality ofconductive sheets which are interleaved with insulator strips, toproduce rotatable capacitance plates embedded in the rotatable headassembly. The head assembly is mounted to a fixed or movable carriage bybearing assemblies which incorporate rotary contacts for conveying thecapacitance electrical signals to an appropriate circuit or circuits forprocessing the signals. The carriage may be mounted to a transportmechanism for transporting the entire head assembly relative to the filmsurface.

It is a principal object and advantage of the present invention toprovide a capacitance thickness measuring device for contacting a movingplastic film to obtain film thickness measurements.

It is another object of the present invention to provide a filmthickness measuring device which contacts a film surface withoutscratching or otherwise damaging the film surface.

It is a further advantage and object of the present invention to providea film thickness measuring device which provides highly accuratethickness measurements through rotatable contact with a moving film inspeed synchronization with the film.

Other and further objects and advantages of the invention will becomeapparent from the following specification and, claims, and withreference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an isometric view of one form of the invention;

FIG. 2 shows the device of FIG. 1 in partial cross section;

FIG. 3 shows a cross-sectional view taken along the lines 3--3 of FIG.2;

FIG. 4 shows an expanded view of a portion of FIG. 2;

FIG. 5 shows a cross-section view of an alternative form of theinvention;

FIG. 6 shows a view taken along the lines 6-6 of FIG. 5;

FIGS. 7A, 7B and 7C show the respective capacitance elements used toconstruct the apparatus of FIGS. 5 and 6;

FIG. 8 shows an exploded isometric view of the capacitance elements ofFIGS. 7A, 7B, 7C;

FIG. 9A shows several rotational positions of the sensor of FIG. 5; and

FIG. 9B shows the electrical signal relationships which are producedfrom the sensor positions of FIG. 8A.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an isometric view of one form of the invention. Acapacitance sensing assembly 10 is attached to a transport mechanism 12by means of a pivotal connector 11. The transport mechanism 12 may allowfor transverse movement of the sensing assembly 10 in the directionsindicated by arrow 13. A carriage 14 may be affixed to transportmechanism 12, carriage 14 holding a rotatably-mounted sensor 16. Thesensor 16 is positioned in contacting proximity to a movable plasticfilm sheet 20, the film sheet 20 being movable in the direction shown byarrow 21. The contacting relationship of sensor 16 with sheet 20 causesrotation of sensor 16 in the direction indicated by arrow 17. Sensor 16incorporates a sensor head 18 which will be described in more detailhereinafter.

FIG. 2 shows a partial cross-sectional view of the device of FIG. 1.Carriage 14 comprises a pair of downwardly-depending legs 14a and 14bwhich are preferably made from a nonconductive material such as aplastic material which is sold under the trademark "Delrin." Carriage 14may be pivotally connected to transport mechanism 12 by means of apivotal connector 11 in such case, a bearing 11a is preferablypositioned to permit relative rotation of carriage 14 with respect totransport mechanism 12. Pivot connector 11 enables carriage 14 toautomatically align itself with the longitudinal movement of film sheet20. Legs 14a and 14b each have respective passages and cavities to passconductors therethrough, and to affix the ends of shafts which arecoupled to sensor 16. A first conductor 22 passes through a passage inleg 14a, conductor 22 being referred to herein as a "positive"conductor, which is electrically connected to a rotary contactor 32.Shaft end cap 24 is mechanically affixed to leg 14a by means of a setscrew 25. Similarly, a "negative" conductor 23 passes through leg 14band is electrically connected to a rotary contactor 34, and end cap 28is mechanically affixed to leg 14b by a set screw 29. Shafts 24 and 28respectively form a part of a rotary contactor 32 and 34. Rotarycontactors 32 and 34 are commercially available devices which providesealed mercury rotary contacts to a rotating mechanism, such as a devicewhich is sold under the trade designation " Rotocon-M1," by MeridianLaboratory of Middleton, Wisconsin. The particular advantage of thisdevice is that it enables a reliable electrical connection to be madebetween a rotating member and a filed member, through the use of sealedmercury rotary contacts. In the particular application described herein,shafts 24 and 28 are fixed members and rotary contactors 32 and 34 arerotatable members affixed inside of rotatable sensor 16. Shafts 24 and28 form a part of the rotary contact device, enabling electricalconnection between rotatable members and the respective conductors 22and 23.

A magnet 27 may be embedded into one of the housings 36, 37 to rotatetherewith. A magnetic pickup 31 is correspondingly positioned in theadjacent leg 14a, 14b, to detect the rotational movement of magnet 27.An electrical signal will be generated in magnetic pickup 31 which maybe passed to a sensor external to the device via wire 33. This mechanismpermits the rotational position of sensor 16 to be identified; morespecifically, it enables the specific capacitive elements within sensor16 to become uniquely identified. Magnet 27 and magnetic pickup 31therefore provide an indexing of the rotational position of sensor 16.If a plurality of capacitive elements are designed into sensor 16, thisindexing scheme enables each capacitive element to be uniquelyidentified. To the extent that different capacitive elements and sensor16 produce different capacitive signals, the indexing scheme enables thedifferent signals to be electronically averaged or balanced, orotherwise compensated for in external circuitry.

Sensor 16 is formed from two split housings 36 and 37. Housing 36 isaffixed to rotary contactor 32 by means of a threaded fastener 38. Splithousing 37 is affixed to rotary contactor 34 by means of a threadedfastener 39. The two housing halves 36 and 37 are held together by meansof threaded fasteners 40. The sensor head 18 is compressed between thetwo housing halves 36 and 37 by virtue of the threaded fasteners 40. Acentral wire 42 projects from the end of rotary contactor 32, and isaffixed to the plurality of positive plates which make up sensor head 18(see FIGS. 3 and 4). These positive plates are designated by the numeral44, and are electrically connected to positive conductor 22 via wire 42and rotary contactor 32. A wire 46 connects a plurality of negativeplates 48 comprising sensor head 18 to rotary contactor 34. The negativeplates 48 are electrically connected to negative conductor 23 via rotarycontactor 34. A plurality of dielectric insulator rings 50 areinterleaved between positive and negative plates, to electricallyinsulate respective adjacent plates, and to provide a known dielectricmaterial between the plates. The outer diameter of insulator plates 50is slightly smaller than the respective outer diameter of positiveplates 44 and negative plates 48.

FIG. 5 shows a cross-section view of an alternative embodiment of thesensor, designated sensor 56. Sensor 56 is rotatably mounted betweenlegs 14a and 14b by rotary contactors 72 and 74, which are equivalent tothe earlier-described rotary contactors 32 and 34. FIG. 6 shows across-section view taken along the lines 6--6 of FIG. 5, wherein sensor56 is formed of two housing halves 76 and 77 which are affixed togetheralong a diameter by fasteners 80. A sensor head 58 comprised of aplurality of plates is compressed between sensor housing halves 76 and77.

The construction of sensor head 58 is illustrated in FIGS. 7A through7C, and FIG. 8 wherein a plurality of negative electrode plates 88 and aplurality of positive electrode plates 84 are arranged in stackedformation, with a plurality of insulator plates 90 interleaved betweenadjacent positive and negative electrode plates. The respective positiveand negative electrode plates each have an arcuate bend proximate thecenter point, and a hole for affixing a wire connection to therespective plates. A wire 82 connects the respective positive plates tothe rotary contactor 72, and a wire 86 connects the respective negativeplates to the rotary contactor 74. The thickness of the plates 84, 88and 90 is considerably exaggerated in FIGS. 6 and 8 for illustration; inpractice the plates would each be about 20 millimeters in thickness.

The respective alternative embodiments of the sensor each providedifferent advantages and capabilities. For example, the embodimentillustrated in FIGS. 1-4 has the capacitive electrodes aligned parallelto the direction of travel of the plastic film material. This providesthe highest possible cross web resolution, so that the sensor may betransported transversely across the web to provide a member of discreetfilm thickness measurements. The sensor construction shown in FIGS. 5-8has the capacitive electrodes aligned perpendicular to the direction oftravel of the plastic film material, thereby providing the maximum downweb resolution. A sensor constructed in this manner will providethickness measurements at discreet down web positions of the movingplastic film material.

FIG. 9A shows five different rotary positions (16a-16e) for sensor 16,wherein in each position the positive plate 44 is rotated to a differentangular position relative to a plastic sheet 20. The plastic sheet ispresumed to be moving in the direction of arrow 21, and the respectiverepresentations of sensor 16 will therefore each rotate in the directionof arrow 17. For convenience, the end points of positive electrode 44have been respectively identified with a "1" and a "2."

FIG. 9B shows a number of different electrical signals which aregenerated by the respective sensor positions represented in FIG. 9A, inconjunction with conventional electronic circuits which may be used todevelop digital representations of sensor signals for furtherprocessing. Each signal event is illustrated in a time sequence whichcorresponds to the sensor 16 position shown immediately above the signaldiagram of FIG. 9B. For example, the leftmost sensor 16 position shownin FIG. 9A (16a) shows that neither positive electrode plate is incontact with film 20. Therefore, no electrical signal will be generatedby sensor 16a in the position shown. The sensor position shown by 16bplaces electrode "1" in direct contact with moving film 20, and resultsin the generation of voltage signal 110 as illustrated in FIG. 9B.Voltage signal 110 is an analog voltage signal, and the signal may bepassed through an analog "peak and hold" circuit to produce the signal112, which is a signal having a fixed and extended amplitude equal tothe maximum amplitude of signal 110. The peak amplitude of signal 112may be used to provide trigger signal 114, which may be used as thetriggering event to initiate signal 118 to control an analog-to-digitalconverter circuit (A/D), which will enable the conversion of the peakvalue of signal 112 into a digital representation for subsequentprocessing in a digital computer. The time required for the A/Dconversion process to occur is represented by signal 118, the trailingedge of which can be controlled by signal 116, to generate a resetsignal 120 to initialize the circuitry for reception of the nextsubsequent signal.

The sensor position shown by 16c generates no signal, because theelectrodes "1" and "2" are not in contact with film sheet 20. However,the sensor position 16d places electrode "2" in contact with film 20 andgenerates the sequence of signals shown in FIG. 9B, which areessentially identical to the sequence of signals illustrated withrespect to 16b. The sensor position 16e also generates no furthersignals because the electrodes are not in contact with the film surface.

The foregoing signal representations may be utilized in drivingconventional circuits which are well known in the art, to produce adigital representation of the magnitude of the signal sensed by sensor16, which can be readily converted to a value representative of filmthickness.

In operation the rotatable sensor 16 is placed in contactingrelationship against a moving film surface, and the contact of the filmagainst the sensor causes the sensor to rotate in coincidence with thefilm. The rate of film travel and the rotational speed of the sensor areidentical, so the sensor cannot impart scratching against the film as aresult of relative differences in rate of travel therebetween. Incertain embodiments it may be desirable to impart a transverse motion tothe overall sensor assembly, while permitting freely-rotatable motion ofthe sensor relative to the longitudinal travel of the film. In suchembodiments the pivotal movement of the carriage permits transversemovement of the sensor at an angle relative to the film, and minimizesany tendency toward scratching of the film surface. The transverse rateof travel of the sensor head is considerably slower than thelongitudinal rate of travel of the film, so that one pass of the sensorhead across the transverse dimension of the film is accomplished duringa fairly long longitudinal passage of film material.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof, and it istherefore desired that the present embodiment be considered in allrespects as illustrative and not restrictive, reference being made tothe appended claims rather than to the foregoing description to indicatethe scope of the invention.

What is claimed is:
 1. An apparatus for measuring thickness of a movingsheet of film material, comprising:a) a frame mounted transverse to thedirection of movement of said sheet of film material; b) a carriagemounted to said frame, said carriage having a pair of spaced apart legsdirected toward said sheet of film material; c) a rotatable sensor headmounted between said spaced apart legs, said sensor head having acircular cross section contacting said film material, said circularcross section including a plurality of capacitor plates with interleaveddielectric insulators; and d) means for electrically coupling saidcapacitor plates to circuits for detecting changes in capacitance causedby changes in relative thickness of said moving sheet of film material.2. The apparatus of claim 1, further comprising means for pivotallymounting said carriage to said frame.
 3. The apparatus of claim 1,wherein said sensor head further comprises a generally cylindrical bodyhaving said plurality of capacitor plates aligned along the axis of saidgenerally cylindrical body.
 4. The apparatus of claim 1, wherein saidsensor head further comprises a generally cylindrical body having saidplurality of capacitor plates aligned parallel to the axis of saidgenerally cylindrical body.
 5. The apparatus of claim 1, furthercomprising a rotary contactor on respective ends of said sensor head,each of said rotary contactors being connected to one of said spacedapart legs.
 6. The apparatus of claim 5, wherein said rotary contactorsfurther comprise means for conveying electrical signals from saidcapacitor plates.